This application claims priority under 35 USC §119 to Korean Patent Application No. 10-2008-0058047, filed on Jun. 19, 2008 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates generally to image sensors, and more particularly to fabrication of a complementary metal-oxide semiconductor (CMOS) image sensor having an improved spatial signal to noise ratio (SNR).
2. Background of the Invention
An image sensor converts an optical image into electrical signals. Image sensors are now widely used in electronic devices such as digital cameras, camcorders, personal computer (PC) systems, game devices, security cameras, medical micro cameras, or robots due to recent developments in the computer and communication industries.
An image sensor is classified as one of a charge coupled device (CCD) image sensor or a complementary metal-oxide semiconductor (CMOS) image sensor. Generally, a CMOS image sensor has simple operations and is compatible with CMOS process technology for reduced manufacturing cost.
A CMOS image sensor may be implemented with various types of structures. For example, a CMOS image sensor may be implemented with each unit pixel having four transistors for each photodiode. Each photodiode converts received light into an amount of electric charge corresponding to the intensity of the received light. A transfer transistor transmits the accumulated electric charge from the photodiode to a floating diffusion region. When a select transistor is turned on, a drive transistor configured as a source follower amplifier outputs a voltage in response to an electric potential at the floating diffusion region.
For improved display quality of the CMOS image sensor, a high spatial signal to noise ratio (SNR) is desired. For improving the SNR, a respective threshold voltage of the select transistor is desired to be lower than a respective threshold voltage of the drive transistor. If the threshold voltage of the select transistor is higher than the threshold voltage of the drive transistor, the output signal from the drive transistor is disadvantageously reduced resulting in reduced gain and reduced spatial SNR.
In addition, the select transistor and the drive transistor of the CMOS image sensor are manufactured according to general CMOS fabrication processes. In the prior art, a channel ion implantation is performed for simultaneously determining the threshold voltages of the select transistor and the drive transistor. Accordingly, the select transistor and the drive transistor typically have same threshold voltages in the prior art.
FIG. 13A shows a plot of an output voltage Vout and a gain with respect to an input voltage Vin of a conventional image sensor having a select transistor and a drive transistor with the same threshold voltages such as Vth1. Further in FIG. 13A, as the input voltage Vin is increased, the output voltage Vout is increased according to a first curve V11 corresponding to a first gain curve G11.
Alternatively, if the select transistor and the drive transistor have same threshold voltages Vth2 lower than Vth1, as the input voltage Vin is increased, the output voltage Vout is increased according to a second curve V12 corresponding to a second gain curve G12. In that case in FIG. 13A, the gain curve G12 is widened at the lower region of the voltages Vin and Vout but not at the higher region of the voltages Vin and Vout. Accordingly, the spatial signal to noise ratio (SNR) is not increased significantly.
However, a CMOS image sensor having increased spatial signal to noise ratio (SNR) is desired with low manufacturing cost of the CMOS image sensor.